Atlantic pH Crossovers
System Information
The purpose of this analysis was to determine if any significant systematic offset existed between the various legs of the WOCE/NOAA/JGOFS Atlantic Ocean measurements of pH. Several different methods of measurement were used, and data was reported on different reference scales. The most precise Atlantic pH measurements used a spectrophotometric method (Clayton and Byrne 1993), with m-cresol purple as the indicator and either scanning or diode array spectrophotometers; these were performed on A16N and A05R. For most cruises (A12B, A07, A06, A17, A14, A20, A22, and A24) pH values were derived from potentiometric measurements with a glass electrode.
Some of the pH values were reported on the total hydrogen scale, while others were reported on the seawater scale. The seawater scale considers the interaction of hydrogen ions with bisulfate and fluoride ions in seawater, while the total scale includes only the bisulfate contribution (Dickson and Riley 1979; Dickson and Millero 1987). The two scales are linked by the following equation:
pHSWS = pHT - log {(1+[SO42-]T/KHSO4 + [F-]T/KHF)/(1 + [SO42-]T/KHSO4)},
where pHT is a hydrogen ion concentration on the total hydrogen scale, [F-]T and [SO42-]T are the total concentrations of fluoride and sulfate in seawater, and KHF and KHSO4 are the dissociation constants of hydrogen fluoride and sulfate in seawater (Dickson and Riley 1979). For the crossover analyses all total hydrogen scale pH values were converted to the seawater scale (pHSWS)
Crossover Analysis
The stations selected for each crossover were those with carbon data that were that were within roughly 100 km (≈1˚ latitude) of the crossover point. Data from deep water (>1500 m) at each of the crossover locations were plotted against the density anomaly referenced to 4000 dB (σ4) and fit with a second order polynomial. The difference and standard deviation between the two curves was then calculated from 10 evenly spaced intervals over the density range common to both sets of crossovers. The average of the absolute value of the differences of pH for 21 crossovers at 8 locations was -0.0092±0.0086.
Summary
The crossover comparisons suggest good precision, but because of the small number of comparisons available, no further statistical analysis was done with this carbon parameter.
Summary Table of Crossover Results (opens a new window)
References
- Clayton T. and Byrne, R. H., 1993. Spectrophotometric seawater pH measurements: total hydrogen ion concentration scale calibration of m-cresol purple and at-sea results., Deep-Sea Research, 40:2115-2129.
- DeValls, T. A. and A. G. Dickson (1998) The pH of buffers based on 2-amino-2-hydroxymethyl-1,3-propanediol ("tris") in synthetic seawater. Deep-Sea Research 45:1541-1554.
- Dickson, A.G., and F.J. Millero (1987): A comparison of the equilibrium constants for the dissociation of carbonic acid in seawater media. Deep-Sea Research, 34:1733-1743.
- Dickson, A.G. and J.P. Riley (1979) The estimation of acid dissociation constants in seawater media from potentiometric titrations with strong base: I. The ionic product of water-KW. Marine Chemistry 7:89-99.
- Lee, K., F. J. Millero, R. H. Byrne, R. A. Feely and R. Wanninkhof (2000) The Recommended Dissociation Constants of Carbonic Acid for Use in Seawater. Geophysical Research Letters 27:229-232.
- McElligott, S., R.H. Byrne, K. Lee, R. Wanninkhof, F.J. Millero, and R.A. Feely (1998) Discrete water column measurements of CO2 fugacity and pHT in seawater: A comparison of direct measurements and thermodynamic calculations, Marine Chemistry, 60:63-73.
